1. Field of the Invention
This invention relates to the generation of electrical energy by the movement of a magnet within a coil, and more particularly to an ultra low friction bearing for the magnet.
2. Description of the Related Art
Moving a magnet through a conductive coil induces an electromotive force that generates a current in the coil. If the magnet is moved back and forth in a reciprocating motion, the direction of current flow in the coil will be reversed for each successive traverse, yielding an AC current.
Several electrical generating systems have been developed that make use of reciprocating magnet movement through one or more coils. For example, in U.S. Pat. No. 4,260,901, wave motion in a body of water causes a float to move up and down, which in turn imparts a reciprocating motion to a magnet that travels between a pair of coils stationed at opposite ends of its path. In U.S. Pat. No. 5,347,186, a rare earth magnet and a coil are positioned to move linearly back and forth relative to each other. The magnet can either be fixed and the coil moved up and down relative to the magnet, as by wave action, the coil fixed and the magnet moved relative to the coil as by pneumatic pressure, or the coil housing shaken or vibrated, as by being carried by a jogger, to cause a reciprocating motion of a magnet which moves within the coil. In U.S. Pat. No. 5,818,132, a moving magnet is confined to a bi-directional linear or near linear motion through each of at least two mutually spaced coils for providing power in applications such as long life flashlights, alarms systems, communication devices located at places where conventional electric power sources are unavailable, and relatively high power repetitive forces such as the forces on the heel of a shoe during walking or running.
In each of these applications it is necessary to either hold the magnet or coil stationary and forcefully move the other component, or apply a vigorous shaking or vibrating motion to a housing for the assembly so that the magnet moves relative to the coil. This makes the devices unsuitable for applications in which only a gentle actuating force is available to move the housing, particularly if the movement is in a generally horizontal direction. Thus, in the example of a hand held flashlight with a magnet and coil assembly for supplying power to the bulb, simply holding the flashlight in the user""s hand while walking so that it moves in a shallow, generally horizontal arc corresponding to the user""s hand motion, may be insufficient to cause adequate motion of the magnet relative to the housing.
The present invention seeks to provide a new electrical generator system and method in which frictional forces between a magnet and support structure are so low that relative movement between the magnet and a proximate conductive coil is sufficient for useful electrical power generation, even if the movement is horizontal and only a slight movement is imparted to the support structure.
These goals are achieved with an electrical generator in which a magnet is arranged to move relative to a support structure, the assembly having a critical angle of displacement from a horizontal static position of less than 1 degree, and even of less than 10 minutes. This ultra low friction level can be achieved with a ferrofluid bearing that provides a low friction interface between the magnet and support structure. A conductor, preferably a coil, is arranged with respect to the magnet so that an electrical signal is generated in the conductor and coupled out of the system in response to the magnet moving on its support structure. The ferrofluid preferably has a viscosity less than about 5 centipoise, and in a particular embodiment comprises a light mineral oil medium mixed with isoparaffinic acid.
In one embodiment, the magnet moves within an enclosure and produces a moving magnetic field that generates an electrical signal in a surrounding conductive coil. An air flow path is provided between opposite sides of the magnet which it moves, preferably by the magnet occupying less than the full interior cross-sectional area of the enclosure, to prevent a pressure buildup that could retard the magnet movement. For a hand held device, the enclosure can be curved to generally match the arc of an arm swing. The enclosure can be housed in a buoyant outer enclosure to that the unit generates an electrical signal when the outer enclosure is floating and subjected to wave action, or the unit can be suspended to generate an electrical signal in response to wind. Representative applications include battery charging systems, flashlights, environmental sensors, emergency transmitters and cellular telephones.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.